The invention relates to an optical glass fiber with a primary coating, which, at least partially, consists of organo-polysiloxanes containing acrylic acid ester groups and having alkyl and/or aryl groups bound to silicon. The organopolysiloxanes are liquid at the application temperature and are polymerizable by high-energy radiation.
Optical glass fibers are clad with synthetic materials at the time of their production. These synthetic material coating means have the function of protecting the glass fibers - while maintaining their physical, in particular, optical properties - against mechanical and chemical influences of the environment. To fulfill these complex demands, the glass fibers are generally clad with two coatings of different kind and properties.
The first, i.e. the inner cladding layer (primary coating) in general is soft and rubbery elastic. It serves to maintain the mechanical strength inherent in the glass fibers and protects them against microbends and attenuation. The primary coating, therefore, should have a glass transition temperature of &lt;-40.degree. C. and change its mechanical properties within the application temperature range of -40.degree. to +80.degree. C. as little as possible, so that within these temperature limits a constant attenuation response of the coated glass fibers is ensured. The modulus of elasticity of the coating should, if possible, be .ltoreq.10 N/mm.sup.2. In many cases it is also demanded that the refractive index of the coating substance is higher than that of the outer layer of the glass fiber. If it is taken into consideration that the refractive index n.sub.D.sup.25 of quartz glass lies between 1.45 and 1.46, then for the inner coating substance an index of refraction of &gt;1.46 within the application temperature range is required.
The second, or outer, synthetic material cladding has essentially the function of protecting the glass fiber coated with the first synthetic material layer against mechanical and chemical influences of the environment, in particular, against the effect of moisture and against mechanical abrasion. This coating, therefore, is mechanically harder.
The coating substances are applied on the glass fibers during their production. Since the drawing process can take place at rates of greater than 5 m/s and curable synthetic materials or synthetic preparations are used as coating substances, curing of the synthetic materials or synthetic preparations must take place in less than 1 second, preferentially in less than 0.5 s.
Many types of a two-layer coating are already known. They are primarily UV-curable urethane acrylates, and are used in particular for forming the primary as well as also the secondary coating. Such coatings with urethane acrylates are described for example in DE-AS 34 37 531 and EP-OS 0 111 280. These materials, however, do not combine in themselves the required complex properties. By contrast, within recent years, organo-polysiloxanes modified with acrylic acid ester groups have proven useful. Given a suitable constitution, these polymers yield after UV radiation curing in the required time of less than 0.5 second, soft-elastic coatings of the desired kind.
A glass fiber with such primary cladding is described in EP-Al-0 155 051. The glass fiber here is provided with an enveloping layer with a refractive number, which is higher than that of the outer layer of the glass fiber. The enveloping coating is formed of a curable synthetic material composition, which contains a copolymer, which has as monomeric units dimethylsiloxane and at least one siloxane from the group methyl-phenylsiloxane and diphenylsiloxane. The copolymer itself is a compound having the following composition: ##STR1## in which R.sub.1 and R.sub.8 are alkyl, aryl, and acrylate-containing alkyl groups, R.sub.2, R.sub.3, R.sub.4, R.sub.6, and R.sub.7 are alkyl and aryl groups, and R.sub.5 is an acrylate-containing organic group, the median molecular weight of the polymer molecules lies between 1000 and 1,000,000, and the median fractions of the monomeric units per polymer molecule lie within the following limits: EQU 0.005.ltoreq.p.ltoreq.0.995 EQU 0.ltoreq.q&lt;0.995 EQU 0.ltoreq.r.ltoreq.0.49 EQU 0.ltoreq.s.ltoreq.0.1,
where 0.005.ltoreq.(q+2r).ltoreq.0.955 and the monomeric units can be distributed blockwise, alternating or in any given way in the molecule, and where the copolymer contains at least two acrylate groups per molecule.
The acrylate-containing alkyl group is a group having the formula --R.sub.9 OCOCH.dbd.CH.sub.2, in which R.sub.9 is an alkylene residue, for example the group --(CH.sub.2).sub.n -- with n being.gtoreq.1. The coating materials, consequently, corresponding to this definition are made functional with monoacrylate groups, which are bound through a Si--C bond with the siloxane chain.
In the published international patent application WO 84/00424 silicon acrylates as primary coating materials are described, which have a molecular weight of 500 to 5000 and 2 to 6 monoacrylate groups. These monoacrylate groups are preferentially bound through Si--C bonds with the siloxane chain, since these lead to more hydrolysis-stable products than Si--O--C bonds. Potential hydrolytic degradation would lead through changing mechanical properties - to a change of the attenuation response.
The organo-polysiloxanes modified with acrylic acid ester groups are prepared in the manner that polysiloxanes with reactive side chains are converted with suitable functionalized monoacrylate compounds. Here, the preferred method of preparation is the conversion of hydroxyl, carboxyl, mercapto, or amino functional polysiloxanes with monoacrylate compounds, which contain an isocyanate group, for example with the adduct from 2-hydroxyethyl acrylate and isophorone diisocyanate. Such coating materials, however, cannot be adapted to the demand for increasingly higher curing rates without other desirable properties suffering in an unacceptable manner.
Therefore, an object of the invention is the development of optical glass fibers coated with a primary coating based on organopolysiloxanes containing acrylic acid ester groups and having alkyl and/or aryl groups bound to silicon. Another object is to develop this primary coating in such manner that it is stable to hydrolysis as well as rapidly curable without other required properties like low modulus of elasticity, low glass transition temperature and low viscosity being negatively influenced.